Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices has been accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF excimer laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. From this point of view, EUV lithography, which is an exposure technique employing EUV light having a wavelength further shorter than ArF excimer laser, is expected to be prospective as a next generation exposure technique for 45 nm or below. In this specification, EUV light means a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive indices of substances at such a wavelength are close to 1, whereby it is not possible to use a conventional refractive system like photolithography employing visible light or ultraviolet light. For this reason, for EUV lithography, a reflective system, i.e. a reflective photomask and a mirror, is employed.
A mask blank is a stacked structure for production of a photomask, which has not been patterned yet. In the case of a mask blank for a reflective photomask, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, a multilayer reflective film is usually employed, which has high refractive index layers showing a high refractive index to EUV light and low refractive index layers showing a low refractive index to EUV light alternately stacked so as to increase the light reflectivity for light applied to a layer surface, more specifically, the light reflectivity for EUV light applied to a layer surface. For the absorber layer, a material having a high absorption coefficient for EUV light, specifically, for example, a material containing Cr or Ta as the main component is employed.
The multilayer reflective film and the absorber layer are formed on an optical surface of a glass substrate by using a sputtering method such as an ion beam sputtering method or a magnetron sputtering method. At times of forming the multilayer reflective film and the absorber layer, the glass substrate is held by a holding means. Examples for the means of holding a glass substrate include a mechanical chuck and an electrostatic chuck. However, in view of a problem of particle generation, clamping by an electrostatic chuck is preferably employed, as a means of holding the glass substrate when the multilayer reflective film and the absorber layer are formed, particularly as a means of holding the glass substrate when the multilayer reflective film is formed.
Further, in a mask patterning process or mask handling for exposure, clamping by an electrostatic chuck is employed as the means of holding a glass substrate.
The electrostatic chuck is a technique which has heretofore been employed for clamping of a silicon wafer in a process for producing semiconductor devices. Thus, in a case of a substrate such as a glass substrate having a low dielectric constant and a low conductivity, it is necessary to apply a high voltage to obtain a clamping force equivalent to that required for clamping a silicon wafer, and there is a risk of causing a dielectric breakdown.
In order to solve this problem, Patent Document 1 discloses a mask substrate having a back side coating (conductive film) formed of such material as Si, Mo, chromium oxynitride (CrON) or TaSi other than commonly used Cr, which has a higher dielectric constant and a higher conductivity than glass substrate, as a layer for promoting the electrostatic chucking of the substrate.
However, in the mask substrate disclosed in Patent Document 1, since the CrON film has a low adhesion to the glass substrate, there is a problem that peeling occurs between the glass substrate and the CrON film at the time of forming a multilayer reflective film or an absorber layer with the result that particles are generated. Particularly, in the vicinity of the interface between the electrostatic chuck and the CrON film, peeling of the film tends to be caused by a force applied to the vicinity of the interface between the substrate and the electrostatic chuck, which is produced by rotation of the substrate.
Further, since in the mask substrate disclosed in Patent Document 1, a conductive film is formed on the entire region of one surface including chamfers and side faces of the substrate, the adhesive forces of the film to the chamfers and side faces of the substrate are particularly weak since the conductive film is obliquely formed on the chamfers and side faces, and peeling of the film tends to be caused by warpage of the substrate at a time of clamping by an electrostatic chuck or by contact with an end effecter of a robot arm.
If particles are generated e.g. by such peeling of a conductive film at a time of e.g. clamping by an electrostatic chuck, for example, they are likely to inhibit realization of a high quality EUV mask in the step of preparing an EUV mask from the EUV mask blank by forming a mask pattern by irradiation of electron beams, or inhibit realization of transfer in high precision in the step of exposure by the EUV mask. In the case of pattern transfer by using a conventional transmission mask for exposure, since the wavelength of exposure light is relatively long in a UV region (about 157 to 248 nm), even if concave or convex defects are formed on a mask surface, a critical problem is unlikely caused, and accordingly, generation of particles at a time of film-forming has not been recognized as a major problem. However, in a case of using light having a short wavelength such as EUV light as exposure light, even fine concave or convex defects on a mask surface have a major influence on pattern transfer, and accordingly, generation of particles cannot be ignored.
In order to solve the above problems, Patent Document 2 discloses a substrate with a multilayer reflective film, in which generation of particles e.g. due to peeling of a conductive film at a time of clamping the substrate with a conductive film by an electrostatic chuck, is prevented; a high quality reflective mask blank for exposure having few surface defects due to particles; and a high quality reflective mask for exposure having no pattern defect due to particles.
In order to solve the above problems, the substrate with a multilayer reflective film disclosed in Patent Document 2 comprises a conductive film having a composition varying in the thickness direction of the conductive film so that a substrate side of the conductive film contains nitrogen (N) and a surface side of the film contains at least one of oxygen (O) and carbon (C). With respect to the reason why the conductive film has such a structure, Patent Document 2 discloses that nitrogen (N) contained in the substrate side of the conductive film improves the adhesion of the conductive film to the substrate to prevent the conductive film from peeling, and that nitrogen reduces the film stress of the conductive film thereby to increase the attractive force between an electrostatic chuck and the substrate. Further, at least one of oxygen (O) and carbon (C) contained in the surface side of the conductive film increases the surface roughness of the conductive film to an appropriate level, to increase the attractive force between an electrostatic chuck and the substrate at a time of clamping by the electrostatic chuck, to thereby prevent abrasion between the electrostatic chuck and the substrate. Here, oxygen (O) contained in the conductive film roughens the surface roughness (increases the surface roughness) to an appropriate level, and increases the attractive force between the electrostatic chuck and the substrate, and carbon (C) contained in the conductive film decreases the specific resistance of the conductive film to thereby improve the attractive force between the electrostatic chuck and the substrate, according to this document.
In the substrate with a multilayer reflective film disclosed in Patent Document 2, at least one of oxygen (O) and carbon (C) contained in a surface side of the conductive film produces an appropriately roughened state in the surface of the conductive film, to thereby increase the attractive force between an electrostatic chuck and the substrate at the time of clamping by an electrostatic chuck, and to prevent abrasion between the electrostatic chuck and the substrate. However, there is a problem that if abrasion has already occurred, the presence of large surface roughness tends to cause peeling or chipping off of the film to generate particles. Further, when the surface roughness is large, particles (for example, particles from the material of electrostatic chuck or particles of e.g. Mo or Si which is the film material for the film to be formed) on the electrostatic chuck tend to adhere to the conductive film at a time of electrostatic chucking, and since such particles are hard to be cleaned off, there occurs a problem that these particles drop in subsequent steps (e.g. transfer, cleaning or inspection) to cause new defects.
Further, if the substrate side of the conductive film is CrN, since the content of nitrogen (N) is from 40 to 60 at %, the sheet resistance of the conductive film does not become sufficiently low, and it is not possible to sufficiently increase the clamping force by the electrostatic chuck. As a result, it is not possible to sufficiently increase the contact between the electrostatic chuck and the substrate with a conductive film.
In order to solve the above problems of the substrate with a multilayer reflective film disclosed in Patent Document 2, the present applicant has proposed in Patent Document 3 a substrate with a conductive film for an EUV mask blank, characterized in that the conductive film contains chromium (Cr) and nitrogen (N), the average concentration of N in the conductive film is at least 0.1 at % and less than 40 at %, the crystal state of at least a surface of the conductive film is amorphous, the sheet resistance of the conductive film is at most 27 Ω/□, and the surface roughness (rms) of the conductive film is at most 0.5 nm. Further, the present applicant proposes an EUV mask blank to be prepared by using the substrate with a conductive film, a substrate with a multilayer reflective film for the mask blank, and a reflective mask prepared by using the mask blank.
In the substrate with a conductive film disclosed in Patent Document 3, the surface roughness of a surface of the conductive film is small, which improves the contact with an electrostatic chuck. Further, the sheet resistance of the conductive film is low, which improves the clamping force by the electrostatic chuck. As a result, when the substrate with a conductive film is fixed to an electrostatic chuck and used for producing an EUV mask blank, its contact with the electrostatic chuck is improved. When the contact with the electrostatic chuck is improved, generation of particles by abrasion between the electrostatic chuck and the substrate is prevented.